Principal Investigator / Author(s): Bradford, David F

Contractor: UCLA

Contract Number: A932-139

This study focuses on several species of amphibians as indicators of adverse ecological effects of acidic deposition in the Sierra Nevada. Goals of the study were (1) to provide a basis for evaluating the potential future effects of acidic deposition on aquatic-breeding amphibians in the Sierra Nevada at high elevation and (2) to evaluate evidence that acidic deposition may have been a factor in causing recent population declines of amphibians throughout the Sierra Nevada. We conducted laboratory dose-response studies to determine the sensitivity to low pH and elevated aluminum content of water for early life stages of four species, and we also conducted field surveys to characterize the abundance and associated water chemistry of populations at high elevation. A standard toxicity testing approach (i.e., survival endpoints determined using reconstituted soft water) indicated that amphibians are at little risk from low pH in waters acidified to an estimated extreme of pH 5.0 in surface waters due to acidic deposition. This approach also indicated that amphibians are at little risk from the aluminum levels tested, i.e., 39-80 ug/l. Nevertheless, the possibility exists that observed sublethal effects due to pH as high as 5.25 or elevated aluminum at the above levels, such as reduced growth rate and earlier hatching, may represent significant threats to amphibian populations. However, there are no field data indicating the above aluminum levels are likely to be reached or exceeded during episodes of acidification. We tested the hypothesis that acidification of habitats in the field has resulted in elimination of populations from waters most vulnerable to acidification, i.e., low in pH or ANC, or from waters low in ionic strength, a condition that increases the sensitivity of amphibians to low pH. We surveyed potential breeding sites for two declining and one non-declining species at high elevation within 30 randomly selected survey areas, and compared the above chemical parameters between sites containing a species and sites lacking the species. No significant differences were found that were consistent with the hypothesis, and water chemistry did not differ among sites inhabited by the three species. These findings imply that acidic deposition is unlikely to have been a cause of recent amphibian population declines in the Sierra Nevada.